The present invention relates to magnetic flow meters that sense liquids flowing in industrial process plants. In particular, the present invention relates to electrode circuits in such magnetic flow meters.
Magnetic flow meters utilize an insulated flowtube that carries liquid flowing past an electromagnet and electrodes. The electrodes are sealed in the flowtube to make contact with the flowing liquid. The electrodes sense an electromotive force (EMF) magnetically induced in the liquid, and proportional to flow rate according to Faraday""s law of electromagnetic induction.
Electrical leakage from the electrodes or electrode wiring can give rise to measurement errors in the transmitter output that can go undiagnosed by the operator of the process plant for long periods of time. One technique to address the problem of electrical leakage is to attempt to limit errors due to the electrical leakage. For example, a transmitter circuit with an extremely high input impedance is used to sense the EMF. The wiring between the electrodes and the transmitter is also carefully insulated to avoid leakage or extraneous noise. However, these techniques do not attempt to diagnose or quantify the electrical leakage.
A magnetic flow meter is disclosed that includes a diagnostic circuit indicating a presence of electrical leakage in an electrode circuit in the magnetic flow meter. The diagnostic circuit couples to first and second electrodes in the flowtube and to the flowtube ground. The diagnostic circuit senses a first diagnostic potential between the first electrode and ground, and senses a second diagnostic potential between the second electrode and ground.
The diagnostic circuit generates a diagnostic output as a function of a sum of the first and second diagnostic potentials. The sum of the potentials indicates whether there is electrical leakage.
The flowtube includes an insulated tube adapted to carry a flowing liquid that is coupled to the ground. The flowtube also includes an electromagnet.
A transmitter circuit couples to the electromagnet, the first and second electrodes and the ground. The transmitter circuit generates a transmitter output representing a flow rate of the liquid as a function of a differential potential between the first and second electrodes.
The diagnostic output indicates whether the accuracy of the transmitter output is affected by leakage so that corrective action can be taken.